Bread production process needs dough expansion by baker's yeast. In order to provide bread having a certain quality, strict control of the fermentation of baker's yeast, Saccharomyces cerevisiae is necessary. Baker's yeast cells are exposed to many ambient stresses such as freezing, drying, high temperature and high osmotic pressure in the bread production process (Non-patent reference 1). These ambient stresses weaken fermentation ability of yeast. Therefore, it is desired to establish a technique for the production of a yeast strain having high resistance to ambient stresses. Recently, a bread production method using frozen dough has been rapidly become popular, and it is particularly desired to produce a yeast strain having resistance to freezing stress given by said method.
In the fields of bread production and yeast production, ambient stresses which are given to yeast and restrict the fermentation in bread production have been investigated. Many trials for solving the problems have been done by production of yeast having resistance to these ambient stresses.
On the other hand, as findings on stress-resistant mechanisms have been accumulated, trials for reinforcing resistance have been done by modifying genes relating to resistance.
For example, an amino acid, proline has an action to protect yeast from stresses such as freezing, drying and oxidation (Patent reference 1). It is known that a yeast strain, which lacks a PUT1 gene encoding proline oxidase and expresses a mutant PRO1 gene encoding a gamma-glutamyl kinase variant, accumulates proline in the cell, thereby the strain becomes resistant to ethanol (Patent reference 2).
It is also known that N-acetyl transferase Mpr1 protects yeast from oxidative stresses such as heat shock, hydrogen peroxide treatment, ethanol and low temperature stress (Non patent references 2-4) and that yeast efficiently expressing a mutant Mpr1 has resistance to drying and a high temperature (Patent reference 3).
In addition, it is known, for example: deletion of CBS2 gene causes sensitivity to drying stress (Non patent reference 5); destruction of DBF2 gene causes a high sensitivity to a sorbitol stress (Non patent reference 6); Resistance to high sucrose stress is remarkably lost in a strain in which an aromatic amino acid synthesis-related gene (for example, ARO1, etc.) is disrupted; in a strain in which BUD23, GON7 and SPT20 genes are disrupted, a sensitivity to a high sucrose stress is increased (Non patent reference 7); and in a strain in which OCA1 gene or OCA2 gene (encoding a phosphatase catalyzing dephosphorylation of a phosphorylayted protein), or ALD2 gene is disrupted, resistance to high sucrose stress is increased (Patent reference 4).
Regarding freezing stress, it is known, for example: in a strain in which ATH1 gene encoding an acidic trehalase is disrupted, resistance to freezed dough stress and resistance to high sucrose stress are increased (Patent reference 5); in a strain in which CAR1 gene encoding an arginase is disrupted, amino acids having high polarity are accumulated in the cells and resistance to freezing stress is increased (Patent reference 6); and in a strain in which PMR1 gene or SNF5 gene is disrupted, resistance to freezing stress is lost (Non patent reference 8).